The extraction of oil and gas from reserves situated below the Earth's surface involves the drilling of bore holes from the surface to the reserves. Typically, a drilling tool with a drill bit attached to its lower end is used to drill such holes. The upper end of the drilling tool attached to a drill string or drill pipe, which is attached to a drive assembly at the surface. The drive assembly causes the drilling pipe to rotate which transmits the rotary motion to the drilling tool and the drill bit. As the drilling tool sinks deeper into the ground, extra sections of drill pipe are added to the drill string.
Furthermore, it is known to provide steerable drilling tools. There are various different types of steerable drilling tool and one example is described in detail below. However, steerable drilling tools typically are capable of bending in response to operator instructions so that the direction of the bore can be changed.
Documents GB 2 392 931A, U.S. Pat. No. 6,233,524B1, WO 01/29372A1 and EP 0 806 542A2 all disclose steerable drilling tools.
One example of steerable drilling tools are rotary steerable tools. Whilst a rotary steerable tool may vary in principle, it will generally comprise of a bias or steering unit which exerts a force, either internally on a flexible central shaft or externally on the borehole wall to affect a change in the steering geometry to the desired direction.
In one mechanisation, the drill pipe is connected to a drive unit located at the surface and transmits the rotary motion of the drive unit via the rotary steerable tool to the drill bit. The rotary steerable tool comprises a flexible central shaft which is connected at its top end via the necessary connections to the drill pipe. The bottom end of the flexible shaft is similarly connected to the drill bit. The flexible shaft is supported by two bearing systems, one at either end. The upper bearing is designed to prevent bending of the shaft above it and the lower bearing is typically of the angular contact type and thus allows movement of the shaft above and below it.
Between the two bearings, around the centre of the length of the flexible shaft, is a bend unit that deflects the shaft. Various mechanisms may be implemented to cause the flexible shaft to be deflected to the designated amplitude so as to cause the correct angular deflection of the shaft in the required direction. It will be apparent that the portion of the flexible shaft located below the angular contact bearing will move in the contra-direction to the portion of the flexible shaft located immediately above the bearing in the bend unit. Other rotary steerable designs exist which generate deflection by alternative methods, for example, eccentric pressure pad application.
Alternatively, the rotary steerable tool may be connected to a device known as a mud motor. Fluid, known as mud, is pumped down the drill string into the mud motor which is positioned between the drill string and the rotary steerable tool. An impeller within the mud motor is driven by the movement of the fluid . The impeller is in turn connected to the rotary steerable tool, and thus the drill bit can be rotated.
Rotary steerable tools typically incorporate a reference stabilised housing which is de-coupled, either actively or passively, from the drill string. For example, the outer housing may be restrained from rotating with respect the drill hole walls by a reference stabiliser located along the outer housing. The stabiliser may comprise a plurality of guides, and in particular may be three or four sets of sprung rollers or contact pads which may accommodate over-gauge hole sections. The outer stabilised housing may in fact rotate in the same sense as the drill bit, but at a very slow rate as the system progresses down the hole. The reference stabiliser is designed and operated to ensure that the ratio of drill bit to outer housing turn rate does not exceed a fixed limit.
It can therefore be appreciated that as the drill bit and rotary steerable tool progress along the drilled bore hole, the trajectory of the assembly, and hence that of the borehole, can be controlled. This control is typically actioned and supervised by a drilling operator at the surface or start location of the bore hole.
In addition to operator controlled drilling, it is known to provide automated guidance of drilling tools using closed loop control systems. In order to implement automated guidance of the drilling tool using closed loop control, continuous, accurate information concerning the direction or position of the drill bit is required. In the absence of such information, drilling operator intervention may be required in order to ensure that the drill bit follows the desired bore hole path. However, in the oil and gas industries, the drilling environment can be particularly inhospitable. The vibrations caused by the drilling tool make it difficult to obtain the continuous, accurate information required. Furthermore, these problems are made worse at greater depths. In view of these factors, closed loop control drilling systems are generally difficult to implement in the oil and gas industries.
Document US 2002/0005297 A1 discloses a closed loop control system for use in the drilling of horizontal underground utility lines. Such lines are typically, drilled in soft sub-surface earth and the drilling system is thus not exposed to the same inhospitable environment experienced in the drilling of oil and gas bore holes. In view of this, this document does not address the problems outlined above in relation to providing continuous and accurate results.
Document U.S. Pat. No. 6,233,524 B1 also discloses a closed loop control system. This document is mainly concerned with extending drill life and improving drilling efficiency by taking various measurements relating to operating conditions and operating the drilling tool accordingly. The document also discloses that the system may be implemented as a navigation device. Although the system is designed for use in oil and gas drilling, it does not address the problems associated with obtaining continuous and accurate results.
GB 2 392 931A also discloses a closed loop control system.
In addition to the above disclosures, several techniques for obtaining directional/positional information are known as described in the following.
Measurement While Drilling (MWD) survey tools are located above the rotary steerable tool in the Bottom Hole Assembly (BHA). BHA is the term used to refer to the components and instruments positioned at the bottom of the drill string. The BHA does not necessarily include the drilling tool itself and in the present application the term BHA is used to refer to the components and instruments placed between the drilling tool and the drill string.
Such MWD survey tools comprise magnetometers and inclinometers which provide the drilling operators respectively with azimuthal deviation data (from a reference, e.g. magnetic north) and inclination measurements relating to the portion of bore hole in which the MWD survey tool and the BHA are currently located. When taken together these measurements provide information concerning the trajectory of the bore hole. Typically, the distance of the MWD survey tool from the surface, i.e. the well bore path length, is derived from the length of drill pipe which has been inserted into the well bore behind the MWD survey tool. Thus, the drilling operators are provided with the attitude (azimuth direction and inclination) of the bore hole at a given bore hole length. This information can be used by the drilling operators to guide the rotary steerable drilling tool.
However, there are various problems with the accuracy and latent reaction time of such a set-up. Firstly, given that the rotary steerable tool can be more than 18 feet long, the conventional MWD survey tool is located a considerable distance from the drill bit. Thus, if the drill bit veers off the desired trajectory (for example owing to rock mechanics) the drilling operator remains unaware of this condition until the MWD survey tool reaches the point at, or beyond which the unplanned deviation occurred. At this time the drill bit has progressed considerably along the deflected trajectory. Only at this point is the drilling operator aware that corrective action may be necessary.
MWDs cannot be placed on or near rotary steerable tools as MWDs comprise magnetometers and rotary steerable tools are constructed using magnetically permeable materials. Furthermore, magnetic sensors generally are difficult to operate on or near rotary steerable tools. Rotary steerable tools can be made out of non-magnetic permeable materials, but this is very expensive and generally avoided. Furthermore, even if non-magnetic materials were used in the construction of the rotary steerable tool, the presence of large diameter steel rotating bodies can result in induced electromagnetic forces generating variable, unstable magnetic fields which preclude the use of magnetometers or result in spurious sensor data. Magnetic interference may also result from the control or line currents within the rotary steerable tool. In particular, the system control circuits may create unstable magnetic fields resulting in local disturbances.
Secondly, as MWD survey tools are typically located within the BHA at the lower end of the drill string, while drilling is in progress, the MWD survey tool is subjected to a high degree of vibration and rotary forces. This makes it difficult to obtain accurate continuous survey data while drilling is in progress. Thus, in typical well bore drilling set-ups, drilling is stopped from time to time in order that accurate surveys may be undertaken, normally at pipe connections (typically at 30 m intervals).
Thirdly, the drill string is typically made up of multiple segments of drill pipe with the BHA located at the lower end. The BHA also comprises tubular components of variable cross section, diameter and length. Both the drill string and BHA are limber in nature which enables the drill string to progress along the variable radius curves of the drilled bore hole.
The BHA is normally composed of larger diameter, thicker walled, components, and is less limber than the drill string. In most, but not all, drilling applications, the BHA is stabilised and is nominally held concentric to the central axis of the bore hole. The standard MWD direction tool is in turn centralised within the BHA, thus providing sensor attitude data which can be said to represent the local bore hole axis, but not necessarily that of the newly drilled hole some distance below or ahead of the MWD tool.
The inherent flexibility of the BHA, and specifically, its connection to the rotary steerable system, is a necessary design attribute enabling the steering system to operate quasi-independently of the reaction forces of the BHA above. Hence, the rotary steerable system can be used to deflect the path of the bore hole in any desired attitude and direction.
For the above reasons, MWD survey tools of the type described above are not ideal for use in closed loop control systems.
At Bit Inclination (ABI) sensors (accelerometers) which are located within the outer housing of the rotary steerable tool itself are also known. Such sensors are typically within a few feet of the drill bit and can thus detect relatively quickly any undesired changes in bore hole inclination at or immediately behind the drill bit trajectory and the bore hole axis. However, this sensor configuration does not provide actual azimuthal change. For example, if the drill bit veers from the desired azimuthal trajectory, but maintains the desired inclination, the operator would not be aware of this condition until the MWD survey tool data becomes available for the relevant section of hole. Additionally, the bore hole, at drill bit depth, would have strayed further from the intended trajectory.
For the above reasons, ABI sensors of the type described above are also not ideal for use in closed loop control systems.
Documents US 2002/0005297 A1, U.S. Pat. No. 6,233,524 B1 and GB 2 392 931A were mentioned above in relation to disclosures of closed loop control systems. However, as discussed above, non of these documents address the issues relating to obtaining continuous and accurate sensor readings during the drilling process.
In document US 2002/0005297 A1, the down-hole sensors are positioned in a drill tube which is positioned proximate and rearward of the drilling tool. Positioning the sensors in such a manner has the same draw backs as described above in relation to the MWD. Thus, no solution is provided to the problem of providing continuous and accurate results.
In documents GB 2 392 931A and U.S. Pat. No. 6,233,524 B1, there is no disclosure relating to the problems associated with providing continuous and accurate results, and thus there is no disclosure relating to the positioning of the sensors in order to overcome these problems.
Documents WO 01/29372A1 and EP 0 806 542 A2 both relate to steerable drilling tools, however neither document discloses closed loop control of the direction or position of the drilling tool on the basis of sensors measuring the direction or position of the drilling tool. Neither document highlights the problems associated with the need to provide continuous and accurate results.
Thus, the above described prior art does not disclose any solutions to the problem of providing continuous and accurate sensor measurements for use in automated guidance of a drilling tool using closed loop control. The lack of continuous, accurate information concerning the direction of the drill bit, or reference quality positional information, means that drilling operator intervention is required in order to maintain the drill bit trajectory along the pre-planned well path in such systems.